To address widespread concerns about cancer risks from residential magnetic field exposures, investigators from NCI and from the Children's Cancer Group have conducted a case-control study of childhood acute lymphoblastic leukemia. Recent analyses focused on methodologic issues of exposure metrics, and concluded that repeated spot measurements of static magnetic fields would be necessary to classify bedrooms of subjects as being "in resonance" or "out of resonance", but that a single spot measurement was insufficient. In addition, magnetic fields from appliances were measured to determine if specific types of appliances, statistically linked with childhood leukemia, actually generated higher magnetic field exposures. Magnetic fields from these appliances were not sufficiently elevated to account for the statistical associations. The etiology of brain tumors is poorly understood, and recorded incidence rates have increased dramatically over the past several decades. Whether this increase is, in part, real or is entirely an artefact of improved diagnosis is a controversial issue. Nonetheless, concern has arisen that one or more increasingly common environmental exposures might cause brain cancer. Examples include industrial chemicals, pesticides, food additives, and electromagnetic fields. In response to such concerns, and to advance understanding of environmental, behavioral and genetic causes of brain tumors, we are collaborating with investigators at three U.S. hospitals in conducting a case-control study of malignant and benign brain tumors. Factors under consideration include workplace exposures to chemical agents and electromagnetic fields, use of cellular telephones, dietary factors, family history of tumors, genetic determinants of susceptibility, home appliance use, reproductive history and hormonal exposures, viruses, medical and dental exposure to ionizing radiation, and other aspects of medical history. Key features of the study include its large size, the emphasis on rapid ascertainment of incident cases and interview of study subjects rather than surrogate respondents, the use of detailed, job-specific questions developed by industrial hygienists to ascertain occupational exposures, and the storage of blood samples for future evaluation of inherited susceptibility, biomarkers of exposure, and gene-environment and gene-gene interactions. Our initial report concerned use of cellular telephones. We found no association of cellular telephone use with brain tumors. In response to concerns about cancer risk from non-ionizing radiation of microwave and radio frequencies, an earlier study of cancer mortality among Korean War Naval Service veterans exposed to microwave radiation has been reactivated. Deaths from all diseases and all cancers were significantly below expectation overall, and for the 20,000 sailors with high radar exposure potential compared to the 20,000 sailors with lower radar exposure potential. No adverse health effects appeared attributable to radar exposure during the Korean war.